Method and circuit for regulating the output voltage from a charge pump circuit, and memory device using same

ABSTRACT

A charge pump circuit is regulated to provide a pumped output voltage having reduced voltage ripple. The charge pump circuit generates a boosted output voltage, and includes a plurality of charge pump stages coupled in series, each including an input terminal, an output terminal, a clock terminal, a capacitor, and a switch. The capacitor of each charge pump stage is coupled between the clock terminal and the input terminal, and the switch of each charge pump stage is coupled between the input terminal and the output terminal. The input terminal of a first charge pump stage in the series is coupled to a voltage source and the output terminal of a last charge pump stage in the series is coupled to a pumped voltage output terminal. The switches of all charge pump stages but the last charge pump stage are selectively closed to allow current to flow in a first direction and selectively opened to prevent current flow in a second direction that is opposite the first direction. The switch of the last charge pump stage has a control input that is coupled to a control terminal. A clocking circuit applies first and second complementary digital signals to the clock terminals of the respective charge pump stages with the charge pump stages that receive the first digital signal alternating with the charge pump stages that receive the second digital signal. A control circuit applies a control signal to the control terminal to allow current to flow in the first direction when the absolute value of a pumped voltage at the pumped voltage output terminal has a magnitude that is less than a predetermined value and to prevent current from flowing in the first direction when the absolute value of the pumped voltage at the pumped voltage output terminal has a magnitude that is greater than the predetermined value.

TECHNICAL FIELD

[0001] The present invention relates to voltage generating circuits, and, more particularly, to a method and circuit for regulating a charge pump circuit to minimize the ripple and the power consumption of the charge pump circuit.

BACKGROUND OF THE INVENTION

[0002] In many electronic circuits, charge pump circuits are utilized to generate a positive pumped voltage having an amplitude greater than that of a positive supply voltage, or to generate a negative pumped voltage from the positive supply voltage, as understood by those skilled in the art. For example, in a conventional dynamic random access memory (“DRAM”), a charge pump circuit may be utilized to generate a boosted word line voltage V_(ccp) having an amplitude greater than the amplitude of a positive supply voltage V_(CC), and a negative voltage pump circuit may be utilized to generate a negative substrate or back-bias voltage V_(bb) that is applied to the bodies of NMOS transistors in the DRAM. Another typical application of a charge pump circuit is the generation of a high voltage utilized to erase data stored in blocks of memory cells or to program data into memory cells in non-volatile electrically block-erasable or “FLASH” memories, as will be understood by those skilled in the art.

[0003]FIG. 1 is a schematic of a conventional two-stage charge pump circuit 100 that generates a pumped output voltage V_(p) having an amplitude greater than the amplitude of a supply voltage source V_(cc) in response to complementary clock signals CLK and {overscore (CLK)}, as will be described in more detail below. The charge pump circuit 100 includes two voltage-boosting stages 102 and 104 connected in series between an input voltage node 106 and an output voltage node 108. The voltage-boosting stage 102 includes a capacitor 110 receiving the clock signal CLK on a first terminal and having a second terminal coupled to the input node 106. A diode-coupled transistor 112 is coupled between the input voltage node 106 and a voltage node 114, and operates as a unidirectional switch to transfer charge stored on the capacitor 110 to a capacitor 116 in the second voltage-boosting stage 104. The capacitor 116 is clocked by the complementary clock signal {overscore (CLK)}. A transistor 118 transfers charge stored on the capacitor 116 to a load capacitor C_(L) when the transistor 118 is activated. A threshold voltage cancellation circuit 122 generates a boosted gate signal V_(BG) responsive to the CLK and {overscore (CLK)} signals, and applies the signal V_(BG) to control activation of the transistor 118. When the CLK and {overscore (CLK)} signals are high and low, respectively, the circuit 122 drives the signal V_(BG) low to turn OFF the transistor 118, and when the CLK and {overscore (CLK)} signals are low and high, respectively, the circuit 122 drives the signal V_(BG) high to turn ON the transistor 118. The cancellation circuit 122 may be formed from conventional circuitry that is understood by those skilled in the art. The charge pump circuit 100 further includes a diode-coupled transistor 120 coupled between the supply voltage source V_(CC) and node 106. The diode-coupled transistor 120 operates as a unidirectional switch to transfer charge from the supply voltage source V_(CC) to the capacitor 110.

[0004] A ring oscillator 124 generates an oscillator clock signal OCLK that is applied to a switching circuit 126 coupled between the ring oscillator 124 and a clocking-latching circuit 128. The switching circuit 126 receives a regulation output signal REGOUT from external control circuitry (not shown in FIG. 1), and when the REGOUT signal is inactive low, the switching circuit 126 presents a low impedance and thereby applies the OCLK signal to the clocking-latching circuit 128. When the REGOUT signal is active high, the switching circuit 126 presents a high impedance, which isolates or removes the OCLK signal from the clocking-latching circuit 128. The clocking-latching circuit 128 latches the applied OCLK signal and generates the complementary clock signals CLK and {overscore (CLK)} responsive to the latched OCLK signal. The CLK and {overscore (CLK)} signals have the same frequency as the OCLK signal, and are complementary signals so there is a phase shift of 180° between these signals.

[0005] The operation of the conventional charge pump circuit 100 will now be described in more detail with reference to the timing diagram of FIG. 2, which illustrates the voltages at various points in the charge pump circuit 100 during operation. In operation, the charge pump circuit 100 operates in two modes, a normal mode and a power-savings mode. During both the normal and power-savings modes of operation, the ring oscillator 124 continuously generates the OCLK signal. The charge pump circuit 100 operates in the normal mode when the pumped output voltage V_(P) is less than a desired pumped output voltage V_(PD). When V_(P)<V_(PD), the external control circuitry drives the REGOUT signal inactive low causing the switching circuit 126 to apply the OCLK signal to the clocking-latching circuit 128. In response to the applied OCLK signal, the clocking-latching circuit 128 latches the OCLK and clocks the stages 102 and 104 with the CLK and {overscore (CLK)} signals generated in response to the latched OCLK signal.

[0006] At just before a time t₀, the CLK signal is low having a voltage of approximately 0 volts and the {overscore (CLK)} signal is high having a voltage of approximately the supply voltage V_(CC), and each of the voltages on the nodes 106, 114, and 108 and the have assumed values as shown for the sake of example. Also, before the time t₀ the REGOUT signal is inactive low and the circuit 122 drives the boosted gate signal V_(BG) high responsive to the CLK and {overscore (CLK)} signals being low and high, respectively. When the CLK signal is low, the terminal of the capacitor 110 is accordingly at approximately ground and the voltage at the node 106 is sufficiently low to turn ON the diode-coupled transistor 120, transferring charge from the supply voltage source V_(CC) through the transistor 120 to charge the capacitor 110. As shown in FIG. 2, the voltage at the node 106 (i.e., the voltage across the capacitor 110) is increasing just before the time t₀ as the capacitor 110 is being charged. Also just before the time t₀, the voltage at the node 114 equals the high voltage of the {overscore (CLK)} signal plus the voltage stored across the capacitor 116 (V₁₁₆). This bootstrapped voltage on the node 114 is sufficiently greater than the voltage V_(P) on the output voltage node 108 to turn ON the transistor 118, transferring charge from the capacitor 116 through the transistor 118 to the load capacitor C_(L). As shown, the voltage at node 114 is decreasing and the voltage V_(P) increasing just before the time t₀ as charge is being transferred through the transistor 118.

[0007] At the time t₀, the CLK signal goes high, driving the voltage on the node 106 to the high voltage (V_(CC)) of the CLK signal plus the voltage stored across the capacitor 110 (V₁₁₀). At this point, the voltage on the node 106 is sufficiently high to turn OFF the transistor 120, isolating the node 106 from the supply voltage source V_(CC). Also at the time t₀, the {overscore (CLK)} signal goes low (to ground), causing the voltage on the node 114 to equal the voltage V₁₁₆ stored across the capacitor 116. The voltage on the node 106 is now sufficiently greater than the voltage on the node 114 to turn ON the transistor 112, transferring charge from the capacitor 110 through the transistor 112 to the capacitor 116. As shown in FIG. 2, between the time t₀ and a time t₁, which corresponds to the interval the CLK signal is high and {overscore (CLK)} signal is low, the voltage at the node 106 decreases and the voltage at the node 114 increases as charge is pumped or transferred through the transistor 112. It should be noted that during this time, the transistor 118 is turned OFF because the voltage V_(P) is sufficiently greater than the voltage at the node 114 during normal operation of the charge pump circuit 100.

[0008] At the time t₁, the CLK and {overscore (CLK)} signals go low and high, respectively, and the charge pump circuit 100 operates in the same manner as previously described for just before the time t₀. In other words, the transistor 112 turns OFF and transistors 118 and 120 turn ON, and charge is transferred from the supply voltage source V_(CC) through the transistor 120 to the capacitor 110 and charge is transferred from the capacitor 116 through the transistor 118 to the load capacitor C_(L). As seen in FIG. 2, from the time t₁ to a time t₂ the voltage at the node 106 increases as the capacitor 110 is charging and the voltages on nodes 114 and 108 decrease and increase, respectively, as charge is transferred from the capacitor 116 to the load capacitor C_(L). At the time t₂, the CLK and {overscore (CLK)} signals again go high and low, respectively, and the charge pump circuit 100 operates as previously described at the time t₀.

[0009] The charge pump circuit 100 continues operating in this manner during the normal mode, pumping charge from the supply voltage source V_(CC) to the successive capacitors 110, 116, and C_(L) to develop the desired pumped voltage V_(PD) across the capacitor C_(L). When the pumped output voltage V_(P) becomes greater than the desired voltage V_(PD), the charge pump circuit 100 commences operation in the power-savings mode of operation, which occurs at a time t₃ in FIG. 2. In response to the pumped output voltage V_(P) becoming greater than the desired voltage V_(PD), the external control circuit drives the REGOUT signal active high, causing the switching circuit 126 to present a high impedance so that the OCLK signal no longer clocks the clocking-latching circuit 128 which, in turn, no longer clocks the voltage-boosting stages 102 and 104. As a result, the CLK and {overscore (CLK)} signals remain in their previous latched states until the pumped output voltage V_(P) is less than V_(PD). This is seen in the example of FIG. 2 at a time t₄ when, although the OCLK signal goes high, the CLK and {overscore (CLK)} signals remain low and high, respectively, since OCLK signal is not applied to the clocking-latching circuit 128.

[0010] Once the pumped output voltage V_(P) becomes less than V_(PD), the control circuit drives the REGOUT signal inactive low and the charge pump circuit 100 again commences operation in the normal mode. As will be understood by those skilled in the art, the switching circuit 126 enables the charge pump circuit 100 to very quickly switch into the normal mode of operation since the ring oscillator 124 continually generates the OCLK signal. In other words, since the ring oscillator 124 continuously generates the OCLK signal, transition from the power-savings to normal mode is delayed only by the switching time of the circuit 126, which is very fast. In contrast, if the ring oscillator 124 was turned ON and OFF responsive to the REGOUT signal, the settling time (i.e., the time for the CLK, {overscore (CLK)} signals to stabilize) of the oscillator when turned back ON is much greater than the switching time of the circuit 126. As a result, in this situation the voltage V_(P) could continue to decrease during this settling time, thereby increasing the ripple of the voltage V_(P).

[0011] The power-savings mode of operation reduces the overall power consumption of the circuit 100 since the CLK and {overscore (CLK)} signals do not clock the stages 102 and 104 when the voltage V_(P) is greater than the desired voltage V_(PD). Although the overall power consumption of the circuit 100 is reduced and the switching circuit 126 alleviates some of the ripple introduced by switching between modes, operation in the power-savings mode introduces additional ripple of the pumped output voltage V_(P) due to the transistor 118 in the final voltage-boosting stage 104 remaining turned ON during this mode of operation. More specifically, when the charge pump circuit 100 enters the power savings mode of operation the CLK and {overscore (CLK)} signals have one of two states. If the CLK and {overscore (CLK)} signals are high and low, respectively, when the REGOUT signal goes active to enter the power-savings mode, then the boosted gate signal V_(BG) remains low during this mode and the transistor 118 is turned OFF. In this situation, the turned OFF transistor 118 isolates the output node 108 and the voltage V_(P) is not affected by the voltage on the node 114.

[0012] In contrast, if the CLK and {overscore (CLK)} signals are low and high, respectively, when the REGOUT signal goes active, the transistor 118 may remain turned ON during the power-savings mode thereby coupling the output node 108 to the node 114. As a result, the voltage on the node 114 affects the pumped output voltage V_(P) in this situation. For example, as illustrated in FIG. 2, it is seen that when the REGOUT signal goes high at the time t₃ the CLK and {overscore (CLK)} signals are low and high, respectively, so the signal V_(BG) is high turning ON the transistor 118. As seen after the time t₃, the pumped output voltage V_(P) continues to increase as charge is transferred from the capacitor 116 through the transistor 118 to the load capacitor C_(L). Thus, the pumped voltage V_(P) undesirably increases after time t₃ even though it is already greater than the desired voltage V_(PD), thereby increasing the ripple of the pumped output voltage.

[0013] There is a need for a charge pump circuit having a low power consumption and a reduced ripple of the generated pumped output voltage.

SUMMARY OF THE INVENTION

[0014] According to one aspect of the present invention, a charge pump circuit includes a plurality of charge pump stages coupled in series, each including an input terminal, an output terminal, a clock terminal, a capacitor, and a switch. The capacitor of each charge pump stage is coupled between the clock terminal and the input terminal, and the switch of each charge pump stage is coupled between the input terminal and the output terminal. The input terminal of a first charge pump stage in the series is coupled to a voltage source and the output terminal of a last charge pump stage in the series is coupled to a pumped voltage output terminal. The switches of all charge pump stages but the last charge pump stage are selectively closed to allow current to flow in a first direction and selectively opened to prevent current flow in a second direction that is opposite the first direction. The switch of the last charge pump stage has a control input that is coupled to a control terminal. A clocking circuit applies first and second complementary digital signals to the clock terminals of the respective charge pump stages with the charge pump stages that receive the first digital signal alternating with the charge pump stages that receive the second digital signal. A control circuit applies a control signal to the control terminal to allow current to flow in the first direction when the absolute value of a pumped voltage at the pumped voltage output terminal has a magnitude that is less than a predetermined value and to prevent current from flowing in the first direction when the absolute value of the pumped voltage at the pumped voltage output terminal has a magnitude that is greater than the predetermined value.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a schematic of a conventional charge pump circuit.

[0016]FIG. 2 is a signal diagram illustrating the operation of the charge pump circuit of FIG. 1.

[0017]FIG. 3 is a schematic illustrating a charge pump circuit according to one embodiment of the present invention.

[0018]FIG. 4 is a functional block diagram of a memory device including the charge pump circuit of FIG. 3.

[0019]FIG. 5 is a functional block diagram of a computer system including the memory device of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

[0020]FIG. 3 is a schematic of a charge pump circuit 300 including an isolation circuit 302 according to one embodiment of the present invention. The charge pump circuit 300 includes two voltage-boosting stages 304 and 306 connected in series between an input node 308 and an output node 310. In operation, the isolation circuit 302 operates during a power-savings mode to turn OFF the final voltage-boosting stage 306 when a pumped output voltage V_(P) on the node 310 exceeds a desired value. By turning OFF the final voltage-boosting stage 306, the isolation circuit 302 isolates the output node 310 so that the final voltage-boosting stage 306 does not increase the ripple of the pumped output voltage V_(P) during the power-savings mode, as will be explained in more detail below.

[0021] In the charge pump circuit 300, the voltage-boosting stage 304 includes a capacitor 312 receiving a clock signal CLK on a first terminal and having a second terminal coupled to the input node 308. An NMOS diode-coupled transistor 314 is coupled between the input node 308 and a voltage node 316, and operates as a unidirectional switch to transfer charge stored on the capacitor 312 to a capacitor 318 in the final voltage-boosting stage 306. The capacitor 318 receives a complementary clock signal {overscore (CLK)}, and charge stored on the capacitor 318 is transferred through an NMOS transistor 320 to a load capacitor C_(L) when the transistor 320 is activated. A threshold voltage cancellation circuit 322 generates a boosted gate signal V_(BG) responsive to the CLK and {overscore (CLK)} signals, and applies the signal V_(BG) to control activation of the transistor 320. When the CLK and {overscore (CLK)} signals are high and low, respectively, the cancellation circuit 322 drives the signal V_(BG) low turning OFF the transistor 320, and when the CLK and {overscore (CLK)} signals are low and high, respectively, the circuit 322 drives the signal V_(BG) high turning ON the transistor 320. The charge pump circuit 300 further includes a diode-coupled transistor 324 coupled between a supply voltage source V_(CC) and the input node 308. The diode-coupled transistor 324 operates as a unidirectional switch, transferring charge from the supply voltage source V_(CC) to the capacitor 312. Transistors 324 and 314 are not necessarily in diode-coupled configuration. The gate voltage of these transistors can be generated from other threshold voltage cancellation circuit.

[0022] A ring oscillator 326 generates an oscillator clock signal OCLK that is applied to a switching circuit 328 coupled between the ring oscillator 326 and a clocking-latching circuit 330. The switching circuit 328 receives a regulation output signal REGOUT from a feedback control circuit 332. The ring oscillator 326, switching circuit 328, and clocking-latching circuit 330 operate identically to the corresponding components previously described with reference to FIG. 1, and thus, for the sake of brevity, their operation will not again be described in detail. Moreover, one skilled in the art will understand circuitry that performs the required functions of the ring oscillator 326 and circuits 328 and 330. For example, the switching circuit 328 may be a conventional transmission gates the clocking-latching circuit 330 may include a conventional RS flip-flop circuit and buffer circuitry, and the ring oscillator 326 may be formed from a plurality of inverters connected in series with the output from the last inverter being applied to the input of the first inverter, as will be understood by those skilled in the art.

[0023] The feedback control circuit 332 generates the REGOUT signal in response to the pumped output voltage V_(P) generated on the output node 310. When the voltage V_(P) is greater than a desired value, the feedback control circuit 332 drives the REGOUT signal active high, causing the switching circuit 328 to isolate the OCLK signal from the clocking-latching circuit 330. In contrast, when the pumped output voltage V_(P) is less than the desired value, the feedback control circuit 332 drives the REGOUT signal inactive low, causing the switching circuit 328 to apply the OCLK signal to the clocking-latching circuit 330. In the embodiment of FIG. 3, the feedback control circuit 332 includes a ratio circuit 334 that generates a ratio voltage V_(RP) having a value that is equal to the actual pumped output voltage V_(P) times a gain M. The gain M of the ratio circuit 334 is defined by the value of a reference voltage V_(R) divided by the desired value of the pumped output voltage on the node 310, which is designated V_(PD). The gain M of the ratio circuit 334 functions to scale the pumped output voltage V_(P) such that when the pumped output voltage has the desired value V_(PD), the ratio voltage V_(RP) equals the reference voltage V_(R). One skilled in the art will understand circuitry that can be used to form the ratio circuit 334, such as two resisters connected in a voltage divider with a capacitor in parallel with each resistor.

[0024] A comparator 336 then compares the ratio voltage V_(RP) from the ratio circuit 334 to the reference voltage V_(R) and generates an output in response to this comparison. During operation of the charge pump circuit 300, the actual pumped output voltage V_(P) typically has a value that is either less than or greater than the desired pumped output voltage V_(PD). As a result, the ratio voltage V_(RP) will be either less than or greater than the reference voltage V_(R). The ratio voltage V_(RP) is greater than the reference voltage when the pumped voltage V_(P) is greater than the desired voltage V_(PD), causing the comparator 336 to drive its output active high. In contrast, when the ratio voltage V_(RP) is less than the reference voltage V_(R), indicating the pumped voltage V_(P) is less than the desired voltage V_(PD), the comparator 336 drives its output inactive low. The output of. the comparator 336 is applied through an amplifier 338 to a buffer 340 that generates the REGOUT signal responsive to the amplified output from the amplifier 338. When the output from the comparator 336 is active high, the buffer 340 receives this amplified output from the amplifier 338 and drives the REGOUT signal active high. If the output from the comparator 336 is inactive low, the buffer 340 receives this amplified output from the amplifier 338 and drives the REGOUT signal inactive low.

[0025] In the charge pump circuit 300, the REGOUT signal from the feedback control circuit 332 is further applied to the isolation circuit 302. The isolation circuit 302 is coupled to the gate of the transistor 320 in the final voltage-boosting stage 306. When the REGOUT signal is inactive low, the isolation circuit 302 presents a high impedance to the gate of the transistor 320 and thus the voltage on the gate is determined by the boosted gate signal V_(BG) from the cancellation circuit 322. When the REGOUT signal is active high, the isolation circuit 302 turns ON, coupling the gate of the transistor 320 to approximately ground to thereby turn OFF the transistor 320. In the embodiment of FIG. 3, the isolation circuit 302 includes a load transistor 342 and an enable transistor 344 connected in series between the gate of the transistor 320 and ground as shown. The enable transistor 344 receives the REGOUT signal from the feedback control circuit 332, turning ON and OFF when the REGOUT signal is high and low, respectively.

[0026] In operation, the charge pump circuit 300 operates in two modes, a normal mode and a power-savings mode. During both the normal and power-savings modes of operation, the ring oscillator 326 continuously generates the OCLK signal. In the following description, the means by which each of the voltage-boosting stages 304 and 306 boosts the corresponding voltage is substantially the same as in the charge pump circuit 100 previously described with reference to FIG. 1, and thus for the sake of brevity will not be described in more detail. Instead, the following description will explain the operation of the feedback control circuit 332 and isolation circuit 302 in reducing the voltage ripple of the pumped output voltage V_(P) generated by the charge pump circuit 300.

[0027] The charge pump circuit 300 operates in the normal mode when the pumped output voltage V_(P) is less than the desired pumped output voltage V_(PD). When the actual pumped output voltage V_(P) is less than the desired voltage V_(PD), the ratio circuit 334 develops the ratio voltage V_(RP) having a value that is less than the reference voltage V_(R), causing the comparator 336 to drive its output inactive low. In response to the low output from the comparator 336, the amplifier 338 applies the amplified low output to the buffer 340 which, in turn, drives the REGOUT signal inactive low. In response to the low REGOUT signal, the transistor 344 turns OFF causing the isolation circuit 302 to present a very high impedance to the gate of the transistor 320. In this situation, the value of the boosted gate signal V_(BG) from the cancellation circuit 322 controls the operation of the transistor 320. The low REGOUT signal also causes the switching circuit 328 to present a low impedance, thereby applying the OCLK signal from the ring oscillator 326 to the clocking-latching circuit 330 which, in turn, clocks the voltage-boosting stages 304 and 306 with the CLK and {overscore (CLK)} signals, respectively. The CLK and CLK signals also clock the cancellation circuit 322 during the normal mode of operation. Thus, during the normal mode of operation, the voltage-boosting stages 304 and 306 and the cancellation circuit 322 operate in response to the CLK and {overscore (CLK)} signals to generate the pumped output voltage V_(P) on the output node 310 in the same manner as previously described with reference to FIG. 1.

[0028] When the pumped output voltage V_(P) becomes greater than the desired voltage V_(PD), the charge pump circuit 300 commences operation in the power-savings mode of operation. In response to the pumped output voltage V_(P) becoming greater than the desired voltage V_(PD), the ratio circuit 334 generates the ratio voltage V_(RP) having a value that is greater than the reference voltage V_(R). When the ratio voltage V_(RP) is greater than the reference voltage V_(R), the comparator 336 drives its output active high and this high output is applied through the amplifier 338 to the buffer circuit 340. In response to the amplified high output of the comparator 336, the buffer 340 drives the REGOUT signal active high. In response to the high REGOUT signal, the switching circuit 328 presents a high impedance so that the OCLK signal no longer clocks the clocking-latching circuit 330. As a result, the clocking-latching circuit 330 no longer generates the CLK and {overscore (CLK)} signals to clock the voltage-boosting stages 304 and 306. At this point, the CLK and {overscore (CLK)} signals remain in their previous latched states.

[0029] During the power-savings mode of operation, the active high REGOUT signal turns ON the transistor 344 coupling the gate of the transistor 320 to approximately ground through the load transistor 342 and activated transistor 344. As a result, in the charge pump circuit 300 the transistor 320 in the final voltage-boosting stage 306 is turned OFF during the power-savings mode of operation regardless of the level of the boosted gate signal V_(BG) from the cancellation circuit 322, as will now be described in more detail. As previously described, the cancellation circuit 322 generates the boosted gate signal V_(BG) responsive to the CLK and {overscore (CLK)} signals. Thus, the level of the signal V_(BG) is determined by the latched state of the CLK and {overscore (CLK)} signals when the charge pump circuit 300 enters the power-savings mode of operation responsive to the REGOUT signal going active high. More specifically, if the CLK and {overscore (CLK)} signals are latched high and low, respectively, then the boosted gate signal V_(BG) remains low during the power-savings mode. In this situation, the transistor 320 would normally be turned OFF, but this is now ensured by the isolation circuit 302 driving the gate of the transistor 320 to ground and thereby isolating the output node 310 so that the ripple of the pumped output voltage V_(P) is not affected by the voltage on the node 316.

[0030] If the CLK and {overscore (CLK)} signals are latched low and high, respectively, upon entering the power-savings mode, the cancellation circuit 322 attempts to drive the boosted gate signal V_(BG) high. In the charge pump circuit 300, however, the isolation circuit 302 is turned ON in the power-savings mode responsive to the active high REGOUT signal. More specifically, the transistor 344 turns ON and the isolation circuit 302 presents approximately the resistance of the load transistor 342 between the gate of the transistor 320 and ground, The relatively small resistance of the load transistor 342 presents a large load on the output of the cancellation circuit 322, thereby driving the output of the cancellation circuit 322 and thus the gate of the transistor 320 low. Therefore, although the cancellation circuit 322 would normally apply a high signal V_(BG) to turn ON the transistor 320 in this situation, the isolation circuit 302 drives the gate of the transistor 320 low to ensure that the transistor is turned OFF.

[0031] In the charge pump circuit 300, the isolation circuit 302 turns OFF the transistor 320 isolating the output node 310 from the node 316 so that the pumped output voltage V_(P) is unaffected by the voltage on the node 316 independent of the state of the latched CLK and {overscore (CLK)} signals when the power-savings mode is entered. In this way, the voltage on the node 316 does not increase the ripple of the pumped output voltage V_(P) during the power-savings mode. Thus, relative to conventional charge pump circuits, the charge pump circuit 300 may operate with a lower power consumption and a lower voltage ripple of the voltage V_(P) during the power savings mode of operation. In the charge pump circuit 300, the isolation circuit 302 controls the final voltage-boosting stage 306 to reduce the ripple of the voltage V_(P). One skilled in the art will realize, however, a separate isolation circuit could be utilized to isolate the output node 310 from the final voltage-boosting stage. For example, the final voltage-boosting stage could include a diode-coupled transistor and a separate isolation circuit could then be coupled between the output of this final stage and the node 310 and operate responsive to the REGOUT signal.

[0032] Once the pumped output voltage V_(P) becomes less than the desired voltage V_(PD), the feedback control circuit 332 drives the REGOUT signal inactive low and the charge pump circuit 300 once again commences operation in the normal mode. Note that when the REGOUT signal goes inactive low, the transistor 344 turns OFF causing the isolation circuit 302 to present a high impedance to the gate of the transistor 320 so that the level of the boosted gate signal V_(BG) from the cancellation circuit 322 controls the operation of the transistor 320 during the normal mode.

[0033]FIG. 4 is a block diagram of a dynamic random access memory (“DRAM”) 500 including the charge pump circuit 300 of FIG. 3. The DRAM 500 includes an address decoder 502, control circuit 504, and read/write circuitry 506 coupled to a memory-cell array 508, all of these components being conventional. In addition, the address decoder 502 is coupled to an address bus, the control circuit 504 is coupled to a control bus, and the read/write circuitry 506 is coupled to a data bus. The pumped output voltage V_(P) generated by the charge pump circuit 300 may be applied to number of components within the DRAM 500, as understood by those skilled in the art. In the DRAM 500, the charge pump circuit 300 applies the pumped output voltage V_(P) to the read/write circuitry 506 that may utilize this voltage in a data buffer (not shown) to enable that buffer to transmit or receive full logic level signals on the data bus. The charge pump circuit 300 also applies the pumped output voltage V_(P) to the address decoder 502 which, in turn, may utilize this voltage to apply boosted word line voltages to the array 508. In operation, external circuitry, such as a processor or memory controller, applies address, data, and control signals on the respective busses to transfer data to and from the DRAM 500.

[0034] Although the charge pump circuit 300 is shown in the DRAM 500, one skilled in the art will realize the charge pump circuit 300 may be utilized in any type of integrated circuit requiring a pumped voltage, including other types of nonvolatile and volatile memory devices such as FLASH memories as well as SDRAMs, SRAMS, and packetized memory devices like SLDRAMs. When contained in a FLASH memory, the charge pump circuit 300 would typically receive an external programming voltage V_(PP) and generate a boosted programming voltage that is utilized to erase the data stored in blocks of nonvolatile memory cells contained in the array 508, as will be understood by one skilled in the art.

[0035]FIG. 5 is a block diagram of a computer system 600 including computing circuitry 602 that contains the memory device 500 of FIG. 4. The computing circuitry 602 performs various computing functions, such as executing specific software to perform specific calculations or tasks. In addition, the computer system 600 includes one or more input devices 604, such as a keyboard or a mouse, coupled to the computer circuitry 602 to allow an operator to interface with the computer system. Typically, the computer system 600 also includes one or more output devices 606 coupled to the computer circuitry 602, such output devices typically being a printer or a video terminal. One or more data storage devices 608 are also typically coupled to the computer circuitry 602 to store data or retrieve data from external storage media (not shown). Examples of typical storage devices 608 include hard and floppy disks, tape cassettes, and compact disc read-only memories (CD-ROMs). The computer circuitry 602 is typically coupled to the memory device 500 through appropriate address, data, and control busses to provide for writing data to and reading data from the memory device.

[0036] It is to be understood that even though various embodiments and advantages of the present invention have been set forth in the foregoing description, the above disclosure is illustrative only, and changes may be made in detail, and yet remain within the broad principles of the invention. For example, some of the components described above may be implemented using either digital or analog circuitry, or a combination of both, and also, where appropriate, may be realized through software executing on suitable processing circuitry. Therefore, the present invention is to be limited only by the appended claims. 

1. A charge pump circuit for generating a boosted voltage, comprising: a pumping circuit including a plurality of voltage-boosting stages coupled in series, each voltage-boosting stage having a clock terminal adapted to receive a clock signal and generating a boosted voltage responsive to the clock signal, a final voltage-boosting stage in the series further including a control terminal that selectively receives a disable signal to disable the final voltage-boosting stage from generating a boosted voltage that continues to increase after the disable signal is received; and a control circuit coupled to the control terminal, the control circuit generating the disable signal responsive to the boosted voltage generated by the final voltage-boosting stage being greater than a desired value.
 2. The charge pump circuit of claim 1, further comprising a switching circuit coupled between the control terminal of the final voltage-boosting stage and the control circuit, and the switching circuit operable to apply a reference voltage on the control terminal to disable the final voltage-boosting stage responsive to the control signal.
 3. The charge pump circuit of claim 2 wherein the switching circuit comprises a load transistor having a first signal terminal coupled to the control terminal of the final voltage-boosting stage and an enable transistor coupled between a second signal terminal of the load transistor and a reference voltage source, the enable transistor having a control terminal coupled to receive the control signal.
 4. The charge pump circuit of claim 1 wherein each voltage-boosting stage comprises a transistor having signal terminals coupled between an input node and an output node, and a capacitor having a first terminal coupled to the input node and a second terminal adapted to receive the corresponding clock signal.
 5. The charge pump circuit of claim 4 wherein each transistor comprises an NMOS transistor.
 6. The charge pump circuit of claim 1 wherein a ring oscillator clocks odd voltage-boosting stages of the pumping circuit with a first clock signal and clocks even voltage-boosting stages of the pumping circuit with a second clock signal that is the complement of the first clock signal.
 7. A charge pump circuit, comprising: a plurality of charge pump stages coupled in series, each of the charge pump stages including an input terminal, an output terminal, a clock terminal, a capacitor, and a switch, the capacitor of each charge pump stage being coupled between the clock terminal and the input terminal, and the switch of each charge pump stage being coupled between the input terminal and the output terminal, the input terminal of a first charge pump stage in the series being coupled to a voltage source and the output terminal of a last charge pump stage in the series being coupled to a pumped voltage output terminal, the switches of all charge pump stages but the last charge pump stage being selectively closed to allow current to flow in a first direction and being selectively opened to prevent current flow in a second direction that is opposite the first direction, the switch of the last charge pump stage having a control input that is coupled to a control terminal; a clocking circuit generating first and second digital signals that are the complement of each other, the clocking circuit being coupled to supply the first and second digital signals to the clock terminals of the respective charge pump stages with the charge pump stages that receive the first digital signal alternating with the charge pump stages that receive the second digital signal; and a control circuit coupled to the pumped voltage output terminal and the control terminal of the last charge pump stage, the control circuit applying a control signal to the control terminal to allow current to flow in the first direction when the absolute value of a pumped voltage at the pumped voltage output terminal has a magnitude that is less than a predetermined value and to prevent current from flowing in the first direction when the absolute value of the pumped voltage at the pumped voltage output terminal has a magnitude that is greater than the predetermined value.
 8. The charge pump circuit of claim 7 wherein the control circuit comprises: a ratio circuit coupled to the pumped voltage output terminal and operable to develop a ratio output voltage having a value that is proportional to the pumped voltage; a comparator having a first terminal coupled to the ratio circuit and a second terminal adapted to receive a reference voltage, the comparator operable to compare the ratio output voltage to the reference voltage and developing an active compare signal responsive to the comparison indicating the reference voltage is less than the ratio output voltage and developing an inactive compare signal responsive to the comparison indicating the reference voltage is greater than the ratio output voltage; an amplifier having an input coupled to comparator and generating an amplified compare signal on an output; and a buffer circuit coupled to the output of the amplifier to receive the amplified compare signal, the buffer circuit generating the active control signal responsive to the amplified compare signal indicating the reference voltage is less than the ratio output voltage, and generating the inactive control signal responsive to the amplified compare signal indicating reference voltage is greater than the ratio output voltage.
 9. The charge pump circuit of claim 7 wherein the clocking circuit comprises a ring oscillator.
 10. The charge pump circuit of claim 7 wherein each switch comprises a transistor.
 11. A charge pump circuit, comprising: an input voltage node adapted to receive an input voltage; a plurality of voltage-boosting stages coupled in series between the input node and an output node, each voltage-boosting stage having a clock terminal and a final voltage-boosting stage having a control terminal; a clocking circuit coupled to the clock terminals of the voltage-boosting stages; and a control circuit coupled between the output node and the control terminal of the final voltage-boosting stage.
 12. The charge pump circuit of claim 11 wherein the control circuit comprises: a ratio circuit coupled to the output node and having an output; a comparator having a first terminal coupled to the output from the ratio circuit, a second terminal coupled to a reference voltage source, and having an output; an amplifier having an input coupled to output of the comparator and having an output; a buffer circuit having an input coupled to the output of the amplifier and having an output; and a switching circuit coupled between the control terminal and a reference voltage source, and having a control input coupled to the output of the buffer circuit.
 13. The charge pump circuit of claim 11 wherein the clocking circuit comprises a ring oscillator coupled to the respective clock terminals of the voltage-boosting stages.
 14. The charge pump circuit of claim 12 wherein the switching circuit comprises a load transistor having a first signal terminal coupled to the control terminal of the final voltage-boosting stage and an enable transistor coupled between a second signal terminal of the load transistor and the reference voltage source, the enable transistor having a control terminal coupled to the buffer output.
 15. The charge pump circuit of claim 11 wherein each voltage-boosting stage comprises a transistor having signal terminals coupled between an input node and an output node, and a capacitor having a first terminal coupled to the input node and having a second terminal coupled to the clocking circuit.
 16. A charge pump circuit for generating a boosted voltage, comprising: a pumping circuit including a plurality of voltage-boosting stages coupled in series, each voltage-boosting stage having a clock terminal adapted to receive a clock signal and generating a boosted voltage responsive to the clock signal, a final voltage-boosting stage in the series further including a control terminal that selectively receives a disable signal to disable the final voltage-boosting stage from generating a boosted voltage that continues to increase after the disable signal is received; a control circuit coupled to the control terminal, the control circuit generating the disable signal responsive to the boosted voltage generated by the final voltage-boosting stage being greater than a desired value; a ring oscillator operable to generate a clock signal; a switching circuit having a first signal terminal coupled to the ring oscillator to receive the clock signal, a second signal terminal, and a control terminal, the switching circuit applying the clock signal on the second signal terminal responsive to the control signal going inactive and isolating the clock signal from the second signal terminal responsive to the control signal going active; and a clocking and latching circuit that latches the clock signal applied on the second terminal of the switching circuit and generates complementary clock signals responsive to the latched clock signal, the clocking and latching circuit applying a respective one of the respective complementary clock signals to each of the clock terminals to clock the respective voltage-boosting stages.
 17. The charge pump circuit of claim 16 wherein each voltage-boosting stage comprises a transistor having signal terminals coupled between an input node and an output node, and a capacitor having a first terminal coupled to the input node and a second terminal adapted to receive the corresponding clock signal.
 18. The charge pump circuit of claim 16 wherein the control circuit comprises: a ratio circuit coupled to the final voltage-boosting stage to receive the boosted voltage, the ratio circuit operable to develop a ratio output voltage having a value that is proportional to the boosted voltage and a time delay relative to the boosted output voltage; a comparator having a first terminal coupled to the ratio circuit and a second terminal adapted to receive a reference voltage, the comparator operable to compare the ratio output voltage to the reference voltage and developing an active compare signal responsive to the comparison indicating the reference voltage is less than the ratio output voltage and developing an inactive compare signal responsive to the comparison indicating the reference voltage is greater than the ratio output voltage; an amplifier having an input coupled to comparator and generating an amplified compare signal on an output; and a buffer circuit coupled to the output of the amplifier to receive the amplified compare signal, the buffer circuit generating the active control signal to disable the final voltage-boosting stage when the amplified compare signal indicates the reference voltage is less than the boosted voltage, and generating the inactive disable signal to enable the final voltage-boosting stage when the amplified compare signal indicates the reference voltage is greater than the boosted voltage.
 19. The charge pump circuit of claim 16 wherein the clocking circuit comprises a ring oscillator that clocks odd voltage-boosting stages of the pumping circuit with a first clock signal and clocks even voltage-boosting stages of the pumping circuit with a second clock signal that is the complement of the first clock signal.
 20. The charge pump circuit of claim 16 wherein the clocking and latching circuit latches the clock signal responsive to transitions of the clock signal.
 21. A memory device, comprising: an address bus; a control bus; a data bus; an address decoder coupled to the address bus; a read/write circuit coupled to the data bus; a memory-cell array coupled to the address decoder, control circuit, and read/write circuit; and a charge pump circuit that generates a boosted output voltage on an output node coupled to at least one of the address decoder, read/write circuit, and memory-cell array, the charge pump circuit comprising, a pumping circuit including a plurality of voltage-boosting stages coupled in series, each voltage-boosting stage having a clock terminal adapted to receive a clock signal and generating a boosted voltage responsive to the clock signal, a final voltage-boosting stage in the series further including a control terminal that selectively receives a disable signal to disable the final voltage-boosting stage from generating a boosted voltage on the output node that continues to increase after the disable signal is received; and a control circuit coupled to the control terminal, the control circuit generating the disable signal responsive to the boosted voltage generated by the final voltage-boosting stage being greater than a desired value.
 22. The memory device of claim 21 wherein the memory device comprises a flash memory and the control circuit applies the voltage developed on the output node to erase data stored in blocks of memory cells or to program data into memory cells in the memory-cell array.
 23. A computer system, comprising: a data input device; a data output device; and a processor coupled to the data input and output devices, the processor including a memory device comprising, an address bus; a control bus; a data bus; an address decoder coupled to the address bus; a read/write circuit coupled to the data bus; a memory-cell array coupled to the address decoder, control circuit, and read/write circuit; and a charge pump circuit that generates a boosted output voltage on an output node coupled to at least one of the address decoder, read/write circuit, and memory-cell array, the charge pump circuit comprising, a pumping circuit including a plurality of voltage-boosting stages coupled in series, each voltage-boosting stage having a clock terminal adapted to receive a clock signal and generating a boosted voltage responsive to the clock signal, a final voltage-boosting stage in the series further including a control terminal that selectively receives a disable signal to disable the final voltage-boosting stage from generating a boosted voltage on the output node that continues to increase after the disable signal is received; and a control circuit coupled to the control terminal, the control circuit generating the disable signal responsive to the boosted voltage generated by the final voltage-boosting stage being greater than a desired value.
 24. The computer system of claim 23 wherein the memory device comprises a flash memory and the control circuit applies the voltage developed on the output node to erase data stored in blocks of memory cells or to program data into memory cells in the memory-cell array.
 25. A method of controlling a charge pump circuit to generate a pumped voltage, the charge pump circuit including a plurality of voltage-boosting stages connected in series, the method comprising: coupling the final voltage-boosting stage to a pumped output node; controlling the voltage-boosting stages to generate a pumped voltage on the pumped output node; and isolating the final voltage-boosting stage from the pumped output node responsive to the pumped voltage being greater than a desired value.
 26. The method of claim 25 wherein coupling the final voltage-boosting stage to a pumped output voltage node comprises activating the final voltage-boosting stage of the charge pump circuit.
 27. The method of claim 25 wherein isolating the final voltage-boosting stage from the pumped output voltage node responsive to the pumped voltage being greater than a desired value comprises deactivating the final voltage-boosting stage independent of the act of controlling the voltage-boosting stages to generate a pumped voltage on the pumped output voltage.
 28. The method of claim 25 wherein each of the voltage-boosting stages comprises, but not limited to, a diode-coupled transistor.
 29. A charge pump circuit for generating a boosted voltage, comprising: at least one voltage-boosting stage, each voltage-boosting stage being coupled in series and each voltage-boosting stage having a clock terminal adapted to receive a clock signal and generating a boosted voltage responsive to the clock signal; an isolation circuit coupled between a final voltage-boosting stage and an output node, the isolation circuit including a control terminal adapted to receive a disable signal, the isolation circuit operable to apply a boosted voltage generated by the final voltage boosting stage on the output node responsive to the disable signal being inactive, and operable to isolate the final voltage-boosting stage from the output node responsive to the disable signal being active; and a control circuit coupled to the control terminal, the control circuit generating the active disable signal responsive to the boosted voltage on the output node being greater than a desired value.
 30. The charge pump circuit of claim 29 wherein the isolation circuit comprises an additional voltage-boosting stage including a transistor having a first signal terminal coupled to the output node and a second signal terminal coupled to the final voltage-boosting stage, the transistor receiving the disable signal on a control terminal, and the isolation circuit further including a capacitor having a first terminal coupled to the second signal terminal and a second terminal coupled to a clock terminal to receive a clock signal, the additional voltage-boosting stage generating the boosted voltage on the output node responsive to a control signal applied to the control terminal and the clock signal when the disable signal is inactive, and isolating the final voltage-boosting stage from the output node when the disable signal is active. 